This invention relates to systems for fluid purification, and more particularly, to systems for fluid purification by reverse osmosis, ultrafiltration, and gas separation.
Fluid purification is accomplished by devices which subject feed fluids under pressure to special separation units to produce purified product fluids. The feed fluid may be saline water, in which case the product fluid is fresh water and the fluid purification process is known as reverse osmosis. Alternatively, the feed fluid may be a mixture of two or more gases, in which case one or more gases constitute the product fluid and the fluid purification process is known as gas separation. Yet another feed fluid may be a liquid containing solid particles, colloidal matter, or molecules having molecular weights above 1000 in suspension. In this case, the product fluid is the liquid and the purification process is known as ultrafiltration. Often in ultrafiltration, the solid particles rather than the product fluids are the desired material such as in the de-watering of cheese whey.
The separation units used for reverse osmosis fluid purification contain semi-permeable membranes. Relatively pure product fluid diffuses through the active side of the membrane while the dissolved solutes are held back. The amount of product fluid that diffuses through the membrane is dependent on several factors, one of which is the amount of membrane surface area that is available to the feed fluid. Often these membranes are flat, as shown in U.S. Pat. Nos. 2,456,805 to Jarvis (1969); 3,397,785 to Jarvis (1968); and 3,398,834 to Nuttall (1968). One reverse osmosis fluid purification device that is well-known in the art is comprised of a single spiral wrap membrane element contained within a pressure vessel, as described in U.S. Pat. No. 3,933,646 to Kanamaru (1976). Spiral wrap membrane elements are also well-known for gas separation and ultrafiltration.
Reverse osmosis spiral wrap membrane elements are well known. In one such element, a sheet of porous backing material sandwiched between two sheets of semi-permeable membrane form what is commonly known as a membrane leaf. The sandwiched porous backing material forms a product fluid passageway. A central mandrel is included in most spiral wrap membrane elements which has openings in fluid communication with the product fluid passageway. A second sheet of porous material, commonly known as the brine spacer, is layed upon the membrane leaf and together the leaf and brine spacer are spirally wrapped around the central mandrel to form a spiral wrap membrane element. The brine spacer forms a feed fluid passageway through which feed fluid under pressure flows past the membrane sheets. Product fluid flows through the membrane sheets into the product fluid passageway and spirals inwardly to the central mandrel where it is ejected. Spiral wrap membrane elements may be manufactured with several of these membrane leaves.
There have been many attempts in the past, particularly in the area of reverse osmosis, to maximize the production of product fluid by rolling large diameter spiral wrap membrane elements with a limited number of membrane leaves to obtain large membrane surface areas. These attempts have proven unsuccessful because an excessive pressure drop in the product fluid passageways caused by the very long lengths of the passageways results in an ineffective use of the membrane area. Attempts have been made to alleviate the problem of excessive pressure drop in large diameter reverse osmosis spiral wrap membrane elements. One attempt, as shown on pages 4-16 of Publication PB-223 191 of the Office of Saline Water dated September, 1973, produced a spiral wrap membrane element with many short membrane leaves attached by their sandwiched porous backing material along the length of an extra sheet of porous material, the extra sheet being in fluid communication with the central mandrel. This attempt to produce large diameter spiral wrap membrane elements having short product fluid passageways, denoted the "tributary" approach, proved disappointing in that product fluid production was not as efficient as desired for a given diameter device. This shortfall in product fluid production was due in part to the sacrifice in membrane surface area that had to be made to include the extra sheet of porous material for attachment of the many membrane leaf "tributaries".
Other examples of prior art devices are shown in U.S. Pat. Nos. 3,898,158; to Miller (1975); 3,397,790 to Newby (1968); 3,558,481 to Furgerson (1971); 3,583,907 to Borsanyi (1971); 3,838,776 to Brun (1974); and 3,923,664 to Grover (1975).